Geochemistry and U–Pb protolith ages of eclogitic rocks of the Asís Lithodeme, Piaxtla Suite, Acatlán Complex, southern Mexico: tectonothermal activity along the southern margin of the Rheic Ocean

Recent data indicating that the Piaxtla Suite (Acatlán Complex, southern Mexico) underwent eclogite-facies metamorphism and exhumation during the Devono-Carboniferous suggest an origin within the Rheic Ocean rather than the Iapetus Ocean. The Asís Lithodeme (Piaxtla Suite) consists of polydeformed metasediments and eclogitic amphibolites that are intruded by megacrystic granitoid rocks. U–Pb (zircon) data indicate that the metasediments were deposited after <em>c</em>. 700 Ma and before intrusion of <em>c</em>. 470–420 Ma quartz-augen granite. The metasedimentary rocks contain abundant Mesoproterozoic detrital zircons (<em>c</em>. 1050–1250 Ma) and a few zircons in the range of <em>c</em>. 900–992 and <em>c</em>. 1330–1662 Ma. Their geochemical and Sm–Nd isotopic signature is typical of rift-related, passive margin sediments derived from an ancient cratonic source, which is interpreted to be the adjacent Mesoproterozoic Oaxacan Complex. Megacrystic granites were derived by partial melting of a <em>c</em>. 1 Ga crustal source, similar to the Oaxacan Complex. Amphibolitic layers exhibit a continental tholeiitic geochemistry, with a <em>c</em>. 0.8–1.1 Ga source (<em>T</em>_DM age), and are inferred to have originated in a rift-related environment by melting of lithospheric mantle in the Ordovician. This rifting may be related to the Early Ordovician drift of peri-Gondwanan terranes (e.g. Avalonia) from Gondwana and the origin of the Rheic Ocean.

最新研究数据表明，位于墨西哥南部的阿卡特兰杂岩（Acatlán Complex）中的皮亚特拉岩套（Piaxtla Suite）在泥盆纪-石炭纪发生了榴辉岩相变质作用并经历折返抬升，这指示其形成背景为雷尼克洋（Rheic Ocean）而非伊阿珀托斯洋（Iapetus Ocean）。隶属于皮亚特拉岩套的阿西斯岩石体（Asís Lithodeme）由多变形变质沉积岩与榴辉岩质角闪岩构成，后者被巨斑状花岗岩类岩石侵入。锆石铀-铅（U–Pb）定年数据显示，该变质沉积岩的沉积时代晚于约700 Ma，且早于约470~420 Ma形成的石英眼球状花岗岩侵入事件。该变质沉积岩中赋存大量中元古代碎屑锆石（约1050~1250 Ma），同时含有少量年龄介于约900~992 Ma和约1330~1662 Ma的锆石。其地球化学与钐-钕（Sm–Nd）同位素组成符合裂谷相关被动陆缘沉积物的典型特征，物源来自古老克拉通，经推断为邻近的中元古代瓦哈卡杂岩（Oaxacan Complex）。巨斑状花岗岩由约1 Ga的地壳源区部分熔融形成，其源区特征与瓦哈卡杂岩相似。角闪岩层表现出大陆拉斑玄武岩地球化学特征，其源区地壳模式年龄（T_DM）约为0.8~1.1 Ga，经推断形成于奥陶纪裂谷环境，由岩石圈地幔熔融形成。该裂谷作用可能与奥陶纪早期冈瓦纳周缘地体（如阿瓦隆尼亚地体）从冈瓦纳大陆漂移以及雷尼克洋的形成存在关联。